The demand for semiconductor devices has increased dramatically over the years. Many frequently used electronic devices are made possible because of developments in semiconductor devices. As such devices become smaller, more sophisticated, and less expensive, the marketplace demands increasingly higher circuit densities, increased performance, and lower cost.
Increasing the density of semiconductor devices has been accomplished by reducing associated component area. Photolithography is nearing X-ray energy levels, however, and is encountering limits in further size reductions. Three-dimensional semiconductor-based structures have provided additional opportunities to increase density, but error probabilities are multiplied as the number of stacked layers increases, and statistics of yield quickly become unsatisfactory.
Crystal-based semiconductor devices generally provide better results than other devices, because regular arrays of atoms provide better electron movement than disordered arrays. At the same time, oxides are needed to keep electrons out of certain areas. Crystals cannot easily be stacked on top of amorphous or non-crystalline oxides, because crystal growth on top of a disordered substrate impedes proper registration of new atoms across the surface and often results in polycrystalline material. Additionally, once a device and a layer have been created, it is generally deleterious to subject them to further thermal input, as is often needed when a next layer is being formed. These and other challenges increase the difficulties and costs associated with stacked devices.